The presence of dissolved phosphate in various effluents, such as industrial effluents, wastewater, animal/livestock wastewater, etc., is a long-standing problem in the art. For example, the majority of phosphate ions (PO43−) in anaerobically digested dairy effluent is sequestered in a finely-suspended, intractable calcium-phosphate solid, and is effectively unavailable for practical and efficient recovery by current methods.
Phosphorus (referred to herein as “P”) is a non-renewable resource and an important non-substitutable macronutrient, existing in nature as phosphates in various inorganic or organic forms, and ranging from the simple to the very complex in terms of molecular structure. Because P is essential for all biological processes, there is concern that the current demand and exploitation (total annual production is about 20 million tons of P, derived from roughly 140 million tons of rock concentrates (2)) of this non-renewable resource is not sustainable. Nearly all the P used globally is mined from a relatively small number of commercially-exploitable deposits, and it has been estimated that the global economic P reserves may last about 100 years at the current rate of extraction (1). Therefore, the world's P resources are finite and should be used efficiently and in a sustainable way. Additionally, aside from the non-renewable resource aspect, there is need to improve P management, particularly from the environment protection perspective because, for example, P-enrichment in receiving waters is associated with harmful algae blooms that affect the health and vitality of wetlands and marine environments.
Therefore, there is a pronounced need in the art to develop methods for increasing the life expectancy of the world's limited P resources. There is a pronounced need in the art to develop methods for recovery and recycling of P from, for example, effluents and waste materials. There is a pronounced need in the art to develop methods for more efficient use of P in agriculture, both as fertilizers and animal manures.
One of the main potential sources for P recycling is animal manure because of its high P content, and large amounts of manure are produced annually in the world, particularly from concentrated animal feeding operations (CAFOs). Currently, the dominant management practice is direct land disposal of the manure. However, a significant disadvantage or challenge associated with such direct land disposal of manure is the limited land available for such disposal, and the water contamination that can potentially occur from the excess nutrients that accumulate as a result of long-term manure application (3, 4). One alternative to direct land disposal manure management is the use of anaerobic digestion (AD) technology. AD, with its conversion of the manure's organic carbon to biogas, and AD has become an option for the utilization and treatment of animal manure. However, since AD does not remove any nutrients during the process, the digested effluent still maintains a high sequestered P concentration (5, 6). Ideally, the excess P in the effluent should be recovered before the effluent goes to storage and disposal on land to enable more efficient controlled use of the P, and to preclude contamination of water resources.
Struvite (magnesium ammonium phosphate hexahydrate or MgNH4PO4.6H2O). Generally, art-recognized P removal technologies applied to wastewater include chemical and biological processes. However, biological methods that incorporate P in microbial biomass are not optimal or particularly suitable for animal manure because the high P content would yet produce large quantities of biosolids to be disposed of. Chemical methods include settling, flocculation, precipitation, and electrocoagulation, etc. A recently developing chemical technology for P removal and recovery is crystallization of P in the form of struvite (magnesium ammonium phosphate hexahydrate or MgNH4PO4.6H2O) (7-12). Struvite is crystalline and thus well suited for removal in a crystallizer. In addition, as a granular product struvite is more compact than other chemical precipitates, and it performs well as a slow-release fertilizer. Struvite formation requires reaction between three soluble ions in solution, Mg2+, NH4+ and PO43−, to form precipitates with low solubility (struvite has a pKsp of 12.6). Struvite precipitation is controlled by pH, supersaturation, and presence of impurities, such as calcium 13, 14). High pH (e.g., pH 8.5) and supersaturation of the three ions (Mg2+, NH4+ and PO43) are favorable to struvite formation. Struvite crystallization for P recovery has been successfully demonstrated in lab, pilot, and full-scale models using swine wastewater, and several crystallizer reactors have been designed and operated through this process both in pilot and field scale (16-19). High total P removal (˜80%) was obtained in the field scale tests (18), indicating that this crystallization process can be used for P recovery from swine wastewater.
Unfortunately, however, struvite precipitation has not been proven effective in digested dairy effluent, which has different properties than swine wastewater (20). For example, in the present Applicants' previous work, P removal from dairy effluent was investigated using a struvite crystallizer designed as a cone-shaped fluidized bed reactor, and the crystallizer was used to achieve high P removal (˜80%) from swine wastewater (18). However, surprisingly, poor P removal (<15%) was obtained under various conditions (20) for treating dairy effluent from an anaerobic digester. The results suggested that the P was not available as an ionic form after anaerobic digestion of the dairy manure. Instead, the majority of the P was in a fine suspended solid form.
There is therefore, not only a pronounced need in the art for novel methods for recovery and recycling of P from, for example, effluents and waste materials, but also a pronounced need for novel methods for recovery of P in the form of struvite.